I'm here to assist you in finding the activation code in QuickBooks Pro 2015 and offering an alternative method to download the 30-day free trial for UK QBDT 2021,@john_b1.

To register with QuickBooks Desktop, you can request the activation code from QuickBooks Online Support. Please know that although there is no direct support for QuickBooks Desktop, this option is available for your request.

Meanwhile, you can download and install the temporary version of QuickBooks Desktop 2021, which provides 30 days of access. No license number is required in this downloadable link. 

You need to uninstall the existing version of the desktop before installing the temporary one in its place. Here's how:

1. Log in to your QuickBooks Desktop, and open the company file you want to move to the temporary desktop version.
2. Press F2 to open the Product information window.
3. In the File Information section, please note where you saved your company file.
4. Uninstall QuickBooks Desktop from your computer, then Download and install the temporary version of QuickBooks Desktop that'll give you access for 30 days.
5. Open the new version of QuickBooks Desktop.
6. Go to File, then Select Open, or Restore Company if you don't have your company file open.
7. Select Open company file, and then select Next.
8. If you have a backup company file (QBB) instead of a company file (QBW), select Restore a back up copy.

Moreover, you can check out this article to learn how to back up your accounting data in QuickBooks Desktop: Back up your QuickBooks Desktop company file.

If you have any other questions about QBDT versions, don't hesitate to leave a reply in the comment section. I'll always be here to assist you. Have a nice one.